Metal halide perovskite solar cells (PSCs) in the inverted planar p-i-n configuration often employ phenyl-C61-butyric acid methyl ester (PC61BM) as electron transport layer, onto which Ag is deposited as outer electrode. However, the energy offset between PC61BM and Ag imposes an energy barrier for electron extraction. In this work, to improve the contact quality of this stack, a small organic molecule (2-(1,10-phenanthrolin-3-yl)naphth-6-yl)diphenylphosphine oxide (DPO) as a cathode interfacial material (CIM), inserted between PC61BM and Ag, is introduced. In devices with the indium tin oxide (ITO)/NiOx/methylammonium lead iodide (MAPbI3)/PC61BM/CIM/Ag configuration, it is found that this results in fill factor (FF) and short-circuit current density values (JSC) that are up to ≈34% and ≈1 mA cm−2 higher, respectively, compared to DPO-free devices. Inserting additional thin ZnO nanoparticle layers further improves the contact quality, leading to a power conversion efficiency of 18.2%. Semitransparent PSCs, utilizing DPO as an interlayer buffer layer are also realised. Resultant devices exhibit improved performance compared to DPO-free devices. This proves that DPO withstands the sputtering of ITO, and may thus find application in perovskite-based tandem devices. It is concluded that DPO acts as an excellent cathode modifier, opening new device-engineering opportunities for p-i-n PSCs, especially in their semitransparent implementation.